The funny thing is that gay or straight, the rules for Catholics don't change.
Sex outside of the sacrament of marriage is sinful: with yourself, with anyone else. If it's outside of the sacrament, it's a sin
From the catechism:
2339 Chastity includes an apprenticeship in self-mastery which is a training in human freedom. The alternative is clear: either man governs his passions and finds peace, or he lets himself be dominated by them and becomes unhappy.126 "Man's dignity therefore requires him to act out of conscious and free choice, as moved and drawn in a personal way from within, and not by blind impulses in himself or by mere external constraint. Man gains such dignity when, ridding himself of all slavery to the passions, he presses forward to his goal by freely choosing what is good and, by his diligence and skill, effectively secures for himself the means suited to this end."127
and...
"The deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose."
A pretty simple rule. If you don't like it, then just don't be Catholic.